linux-kernel - RE: [PATCH] Add /proc/pid

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Date:   Fri, 23 Nov 2018 11:14:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kevin Easton' <kevin@...rana.org>,
        Daniel Colascione <dancol@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "timmurray@...gle.com" <timmurray@...gle.com>,
        "primiano@...gle.com" <primiano@...gle.com>,
        "joelaf@...gle.com" <joelaf@...gle.com>,
        "Jonathan Corbet" <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Mike Rapoport" <rppt@...ux.vnet.ibm.com>,
        Roman Gushchin <guro@...com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Dominik Brodowski" <linux@...inikbrodowski.net>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Ard Biesheuvel" <ard.biesheuvel@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        "MatthewWilcox@...172-31-15-78" <MatthewWilcox@...172-31-15-78>
Subject: RE: [PATCH] Add /proc/pid_generation

From: Kevin Easton
> Sent: 22 November 2018 11:20
> 
> On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:
> > This change adds a per-pid-namespace 64-bit generation number,
> > incremented on PID rollover, and exposes it via a new proc file
> > /proc/pid_generation. By examining this file before and after /proc
> > enumeration, user code can detect the potential reuse of a PID and
> > restart the task enumeration process, repeating until it gets a
> > coherent snapshot.
> 
> I see downthread this patch has been withdrawn, but nonetheless I'm
> still curious - does this actually solve the problem?
> 
> It seems to me that a PID could be reused within a scan even if the
> generation number remains the same at the beginning and end of a scan:

Why not allocate a 48bit generation number to each 16bit pid?
Then you have a 64bit 'extended-pid' that can be assumed to never be reused.
Provided enough interfaces are enhanced to support 'extended-pid' values
you'll never get reused values.

	David

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